Polymerization of Butadiene with Novel tert-Amino Group Initiator

Chapter 5

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Polymerization of Butadiene with Novel tert-Amino Group Initiator Toshihiro Tadaki, Iwakazu Hattori, and Fumio Tsutsumi Elastomers Laboratory, Yokkaichi Research Laboratories, Japan Synthetic Rubber Company, Ltd., 100 Kawajiri-Cho, Yokkaichi, Mie, 510, Japan

A study was made on a new initiator N,N-dimethyl-aminobenzyllithium (TD-Li) with the purpose of improving functionality of the initial chain end in anionic polymerization. After reacting N,N-dimethyltoluidine (TD) with n-butyllithium (BuLi), polymerization of 1,3-butadiene was conducted in several solvents. The "Functionality Introduction Ratio" of polybutadiene was influenced by the polarity of the solvents. That is, the higher the polarity, the higher the "Functionality Introduction Ratio." On the basis of on results of a model reaction, in high polarity solvents the reaction between TD and BuLi advanced efficiently, and the polymerization of butadiene advanced by the benzyllithium type initiator (TD-Li). On the other hand, in a solvent having low polarity, because TD-Li formed partially, the polymerization was initiated by the mixed system of TD-Li and BuLi. Furthermore, in the case where TMEDA was added to the reaction system, the "Functionality Introduction Ratio" was improved.

In recent years, lower rolling resistance has become one of the items most talked about in the tire industry. The reason is that "rolling resistance" is a characteristic which is closely related to the fuel economy of cars. Not only original equipment tires which demand good fuel economy, but also replacement tires which boast high grip performance demand better fuel economy these days. Rolling resistance is primarily related to the hysteresis in the tire tread compound. Thus, the carbon black filled vulcanizates having low hysteresis are in demand as materials for the tire tread, in order to lower the rolling resistance. As rubber for low hysteresis compositions, various solution polymerized rubbers (SSBR) of which the chain ends have been chemically modified, have been developed. They were obtained by the reaction between the living reactive chain ends and some modifiers such as tin compound (1-2), isocyanate compound (3) or 4,4'-bis(diethylamino)-benzophenone (4). It is believed that hysteresis characteristics are improved in carbon black vulcanizates using the above SBR because of the following action. That is, the interaction between 62

©1998 American Chemical Society


63 the modified molecular ends and the carbon black heightens, and as the numbers of free ends decrease, the dispersion of carbon black is improved. However, the modifying points of such SBR are only the ends which have completed polymerization. In the case of anionic polymerization, the polymer obtained always has an initiator fragment (residue) at the initiating point. If a functional group can be introduced to the polymerization initiator itself, it will be possible to synthesize "the di functional polymer" which is expected to more strongly interact with carbon black. Even up to now, several studies have been made on initiators having functional groups. For instance, alkali metal amide initiators (5-7) or a tributylstanyllithium initiator (8) correspond to this. In this report, a study is made on a new initiator "N,N-dimethylaminobenzyllithium (TD-Li)" that contains aromatic tertiary amino group, and discussion is made on the experimental results of 1,3-butadiene polymerization based on such an initiator. The reason we introduced aromatic tertiary amino group as substitution group in the initiator is that we thought the said functional group had strong interaction with carbonblack. Actually, it is reported that the carbonblack vulcanizates using polymers modified with the aforementioned 4,4'-bis-(diethylamino)-benzophenone bas shown improved hysteresis characteristics. Therefore, if the initiator containing aromatic tertiary amino group (which is used in this study) effectively initiates polymerization, it is anticipated that the hysteresis loss of the polymers obtained will be lowered significantly.

-* : polybutadiene Scheme 1

EXPERIMENTAL Polymerization of 1,3-Butadiene Using a N,N-Dimethyltoluidine/nButyllithium as Initiator. A l l polymerization reactions were done under a nitrogen atmosphere inside of pressure resistant bottles capped with crown-caps. To various test solutions in which Ν,Ν-dimethyltoluidine (TD; 39.1 mmol) was dissolved, cyclohexane solution of n-butyllithium (BuLi; 39.1 mmol) was added, and reaction was conducted for specified time in a hot water bath set at 60°C. Twenty-five grams (462 mmol) of 1,3-butadiene was added to the reaction solvent, and reaction was conducted for 60 minutes at 60°C. Subsequently, 2-ethylhexanol (78.2 mmol) was added and the polymerization was stopped. After separating the polymer by making it precipitate in methanol, it was dried under reduced pressure in an oven set at 50°C. Characterization of the Polymers. The microstructure of the polymer was obtained by the following two methods. That is the infrared spectroscopy (calculated by Morero's Method, (9)) and the calculation of peak area ratio of ^H-NMR.

64


The molecular weight of the polymer was obtained by gel permeation chromatography (GPC) based on the standard polybutadiene calibration curve. The measurement of GPC was carried out with RI and U V (254 nm) dual detectors. As it is well known, polybutadiene has no aromatic protons. Thus, no peaks are observed in the vicinity of d = 6.7-7.1 ppm, and no U V absorption should exist at 254 nm. Model Experiments for Verifying the Formation Rate of the Initiator Consisting of N,N-DimethyltoIuidine/n-Butyllithium. Similar to the polymerization experiments described above, the reaction was conducted under nitrogen atmosphere inside of pressure resistant bottles capped with crown-caps. To TD (9.37 mmol) dissolved in various test solvents, cyclohexane solution of BuLi (9.37 mmol) was added, and reaction was conducted for a specified time in a hot water bath set to 60°C. By adding a cyclohexane solution of benzylchloride (9.37 mmol), the reaction was quenched. When the reaction solution was left as it was for a while, white precipitation of lithium chloride formed. After washing the reaction solution sufficiently, the white precipitate was dissolved in water and removed. The water layer used for washing was extracted three times with diethyl ether. Then the whole organic layer was washed with saturated brine, and dried on anhydrous magnesium sulfate. The reaction solvent and diethyl ether were removed by reduced pressure distillation, and the reaction product was obtained. The characterization of the reaction product obtained was done by ÏH-NMR measurement and high performance liquid chromatography using THF as the carrier. RESULTS AND DISCUSSION Polymerizations of 1,3-Butadiene in Several Solvents. First of all, the preparation of initiators was done (reaction of N,N-dimethyl-o-toluidine and nbutyllithium) and polymerization of butadiene was conducted in various solvents having different polarity. The solvents used in the study were cyclohexane (CHX: Dielectric Constant = 2.02), diethyl ether (Et20: e = 4.34), and tetrahydrofuran (THF: e = 7.58). All reactions were carried out at 60°C as described above. The results are shown in Table I. The all polymerization reaction advanced in a uniform system where there were no precipitation or deposition. In all cases, high conversions of 75% or above were obtained for the polybutadiene. In Figure 1, the ^H-NMR spectra of polybutadiene obtained in THF by using "TD/BuLi Initiator, " and polybutadiene obtained in the same solution by using only "BuLi Initiator" are shown. The peak close to 7.0 ppm is attributable to aromatic proton, and the sharp peak close to 2.6 ppm conforms to the aromatic dimethylamino group in TD. The IR spectra of polybutadiene obtained in THF by using only "BuLi Initiator," and polybutadiene obtained in the same solvent by using "TD/BuLi Initiator," are shown in Figures 2 and 3. Polybutadiene using "TD/BuLi Initiator" showed an absorption based on tertiary amino group close to 1300 c m ' l and an absorption based on aromatic group close to 750 cm"l. (Figure 3) In Figure 4, G P C elution curves for polybutadiene obtained in THF by using "TD/BuLi Initiator" and polybutadiene obtained in the same solvent by using only

65


yui

/ v u

initiated by BuLi

CH (CH ) —(CH -CH=CH-CH )-( CH - CH •)- H CH=CHo 3

2 3

2

2

2

H3C CH3 \ / N TMS H C^ 3

CH -(CH -CH=CH-CH H CH - CH )- H 2

2

2

2

CH=CH

2

initiated by TD / BuLi system I I I I L 8 7 6 5 4 3 2

J 1

L 0

δ

in ppm

Figure 1. ^H-NMR spectra of polybutadienes.

4600

4000

3000

2000

1000

400

Figure 2. Infrared absorption spectrum of polybutadiene initiated by BuLi Initiator.

1

(cm" )


66

4600

4000

3000

1000

2000

1

400

(cm- )

Figure 3. Infrared absorption spectrum of polybutadiene initiated by TD / BuLi Initiator.

HC ^ CH -{CH -CH=CH-CH M CH - ÇH )- H H C- ^]) CH=CH 3

2

2

2

2

N

3

2

Polybutadiene Initiated by TD / BuLi System

CH (CH ) —(CH -CH=CH-CH M CH - ÇH )- H CH=CH 3

2 3

2

2

2

2

Polybutadiene Initiated by BuLi

->

elution time

Figure 4. GPC charts of polybutadienes.

67 "BuLi Initiator" are shown. In the case of the former, GPC molecular weight can be obtained from U V absorption at 254 nm. On the other hand, in the case of the latter, no U V absorption existed at 254 nm.


Table I. Polymerization of 1,3-butadiene using TD/BuLi initiator in several solvents (a) Electrical Initiator Property Solvent ε

Conv'n

( b )

(%)

(c) Microstructures cis trans vinyl (%) (%) (%)

Mw/M„

(Φ Functionality Introduction Rate (%)

(1) TD / BuLi CHX Et 0 THF 2

2.02 4.34 7.58

100 100 79

30 19 8

52 29 9

19 52 83

1.2 1.1 1.1

0 49 91

2.02 7.58

100 100

31 5

52 6

18 88

1.3 1.2

—

(2) BuLi CHX THF

—

(a) ; C H X = cyclohexane, ET 0 = diethyleter, THF = tetrahydrofuran THF = tetrahydrofuran (b) ; the dielectric constant (c) ; measured by Infrared spectroscopy calculated by Morero's Method (6 ) (d) ; values obtained by Ή-NMR of polybutadiene 2

NMR spectra of Figure 1 and IR spectra of Figures 2 and 3 suggest that in the case of polybutadiene obtained by using TD/BuLi initiator in THF, aromatic tertiary amino groups are introduced into the chain. Furthermore, in the GPC elution curves of Figure 4, the fact that U V absorption existing at 254 nm in case of the polybutadiene obtained by using "TD/BuLi Initiator, " also supports this result. Based on the results of the above spectra of polybutadiene obtained in THF, we conclude as follows: As shown in Scheme 1, benzyllithium type reaction product is formed, and this acts as the initiator, and the polymerization advances. As a result of using several solvents, we noted that the reaction of TD and BuLi was affected by the polarity of the solvents. In other word, in THF solvent which has high polarity, aromatic tertiary amino groups were introduced into more than 90% of the polybutadiene obtained. However, in the case of diethyl ether, the amount of aromatic tertiary amino group introduced into the polymer was about 50%, and in the case cyclohexane was used as the solvent, that was nearly zero. Moreover, the microstructure of polybutadiene obtained by using TD/BuLi as the initiator has changed according to the polarity of the solvents. The molecular weight distributions were narrow with an Mw/Mn value of 1.1-1.2. They were approximately the same as the polybutadiene obtained by using only BuLi initiator.

68


Polymerization of 1,3-Butadiene in a Mixed Solvent of Cyclohexane and Tetrahydrofuran. Next, in order to clarify the effect of solvent polarity on the "Functionality Introduction Ratio," polymerization of polybutadiene was conducted in a mixed solvent of C H X and THF using TD/BuLi as the initiator. Two cases were adopted for the reaction time of TD and BuLi, namely, 60 minutes and 10 minutes. Polymerization time for polybutadiene which followed was 60 minutes. All reactions were done at 60°C. The results are shown in Table II. In Figure 5, the relation between the CHX/THF ratio of the mixed solvent and the "Functionality Introduction Ratio" (values obtained by ^H-NMR) are shown. As the THF ratio increases (that is, as the polarity becomes higher) the "Functionality Introduction Ratio" of polymer increases. Furthermore, even when the same solvents are used, if the reaction time of TD and BuLi is shortened to 10 minutes, the "Functionality Introduction Ratio" decreases. Furthermore, when aromatic tertiary amino groups are introduced into the polybutadiene, U V absorption appears. Thus, by measuring the U V absorption intensity, it should be possible to judge the functionality. In Figure 6, the relation between "CHX/THF ratio of the mixed solvent" and the "Ratio of U V Intensity and RI Intensity of GPC (compensated by the molecular weight)" are shown. This figure shows a similar trend to the preceding Figure 5.

Table II. Polymerization of 1,3-butadiene using TD/BuLi initiator in mixed solvents of cyclohexane and tetrahydrofuran Œ Microstructures 1,21,4(%) (%)

w

Solvent CHX/THF (wt / wt)

Initiator Aging Time (min.)

100/0 99/1 95/5 88/12 75/25 69/31 39/61

60 60 60 60 60 60 60

19 47 69 69 81 82 83

81 53 31 31 19 18 17

1.2 1.1 1.2 1.1 1.1 1.1 1.1

0 11 15 35 93 90 92

100/0 75/25 64/36 53/47 41/59

10 10 10 10 10

22 76 83 86 87

78 24 17 14 13

1.2 1.1 1.1 1.1 1.1

0 42 43 89 100

(a) ; measured by Ή-NMR (b) ; values obtained by Ή-NMR of polybutadiene

Mw/Mn

Functionality Introduction (%)

ο^·

Ο

CB

100

60

οD

40

ο ο

C il

u_ £

ft-

^ —

—~r\- 0

Reaction Time of TD and BuLi

θ

80

nalil ctior


69

/

Ο

o

Ο

;

60 min.

Ο

;

10 min.

20 0

—

100/0

8

0

/

2

60/40

0

4

0

/

6

0

20/80

0

/

1

0

0

CHX / THF ratio in Solvent (wt% / wt%)

Figure 5. Relationship between "Functionality Introduction Ratio" and polarity of solvent.

(0

3

I

ρ

ce

>

-i

*-

7500-

/

5000-

/ 2500

Reaction Time of TD and BuLi

,0

b

Ο

;

60 min.

Ο

;

10 min.

ο —

100/0

8

0

/

2

0

60/40

4

0

/

6

0

20/80

0

/

1

0

0


Figure 6. Relationship between U V Intensity of polymer and polarity of solvent.

70


Analysis of Reaction Product of T D and B u L i . The first reaction shown in Scheme 1 (where TD-Li formed from TD and BuLi) is influenced by the solvent polarity. Thus, it may be hypothesized that the "Functionality Introduction Ratio" of a polymer varies with the polarity of the solvent. Thus, a slight excess of benzylchloride was added to the solvent in which TD and BuLi were made to react. Then the reaction product was analyzed by ^H-NMR. If benzyllithium type compounds (TD-Li) are formed, upon reaction with benzylchloride N,N-dimethylamino-l,2-diphenylethane (I) should be obtained. On the other hand, if a hydrogen of the methyl group bound to the nitrogen atom is replaced by lithium, compound (II) should be formed by the reaction of benzylchloride. In addition, from the reaction of BuLi and benzylchloride, pentylbenzene (III) is obtained. (Scheme II) In Figure 7, the ^H-NMR spectrum of TD (N,N-Dimethyl-o-toluidine) which is the raw material of the initiator is shown. The peak at 2.6 ppm is attributable to the dimethylamino group, and the peak at 2.3 ppm is attributable to a methyl group bound to the aromatic ring. The peak area ratios are 2:1. The IpI-NMR spectrum of the reaction products obtained by using the solvent system of CHX/THF = 100/0 is shown in Figures 8. The peak at 2.6 ppm consists of two kinds of protons. One of them is equivalent to the proton of dimemylarnino group, and another is believed to be the proton of benzyl position of (III). The peak area ratio of dimethylamino group protons (at 2.6 ppm) and the peak of 2.3 ppm equivalent to the protons of the methyl group bound to the aromatics was 2:1, similar to TD. Furthermore, peaks believed to be attributable to an η-butyl group were observed near 0.5-2.0 ppm.

(Ill)

Scheme II


HqC

CHo

Ν CHo TD

Figure 7. IpI-NMR spectrum of N,N-Dimethyl-o-toluidine.

HaC

CH3

w CH

3

CH (CH2) -CH -Q 3

3

2

(111) 0-CH -C

CH (CH )3~C

2

3

2

I

HQC CHO ό

TMSl

\

/

0

Ν CHo TD

J 8

7

I 6

I 5

I 4

I 3

I 2

I 1

I 0

δ in ppm Figure 8. ^H-NMR spectrum of the reaction products obtained in CHX/THF 100/0.


72 The 1 H - N M R spectrum of the reaction products obtained by using the mixed solvent system of CHX/THF = 75/25 is shown in Figure 9. Contrary to the above, the peak corresponding to the methyl group bound to the aromatics had decreased. (The peak area ratio for the peak at 2.6 ppm and 2.3 ppm was 5.15:1.) A new peak which is believed to be attributable to protons of the benzyl position appeared at 2.9 -3.0 ppm. In addition, peaks near 0.5 - 2 . 0 ppm became very small. Furthermore, the peak area ratio of the peak of dimethylamino group (at 2.6 ppm) and the peak of aromatic protons was almost 2:3 (6:9). This peak area ratio is in good agreement with the corresponding protons in the structure of (I). On the basis of the results of the above spectra, the authors conclude as follows: In the solvent system of CHX/THF =100/0 the reaction of TD and BuLi hardly advanced (i.e., TD remained as it is). The only reaction product with benzylchloride was (III). On the other hand, since in the case of mixed solvent system of CHX/THF = 75/25, the methyl group bound to the aromatics of TD decreased, and protons of benzyl position increased, it was believed that the reaction of TD and BuLi advanced, and TD-Li was formed. In addition, the lithiation reaction of a methyl group bound to nitrogen did not occur in the reaction of TD and BuLi. Next, measurements were made with a high performance liquid chromatography (HPLC) on the reaction products obtained by using mixed solvent system of C H X / T H F at varying ratios. In Fig. 10 the HPLC curve for the reaction product of TD and BuLi in a solvent system of CHX/THF = 88/12 is shown. Beside the solvents (CHX) and the peak of TD which is unreacted raw material, two additional peaks were confirmed in Figure. From the respective molecular weight, the peaks are believed to be (I) and (III). The formation ratios were calculated from the peak areas, and the results are shown in Table III. From Table III, it was found that the formation ratio of (I) and the "Functionality Introduction Ratio" obtained by polymerization of butadiene, matched well. From the above experiments, the following was made clear. In the case of a mixed solvent of CHX/THF having low concentration of THF, the TD-Li forms partially, so the polymerization of butadiene begins with a mixed system of TD-Li and BuLi. As the concentration of THF rises, the reaction product of TD-Li increases, and the "Functionality Introduction Ratio" of polymer improves. ?

?

Addition effect of Ν , Ν , Ν , N -Tetramethylethylenediamine. Based on the above results, in order to introduce aromatic tertiary amino groups into the polybutadiene efficiently, the authors found that it was effective if reaction was conducted in a solvent having high polarity. However, the microstructure of polybutadiene thus obtained is also influenced by the polarity of the solvent. The higher the polarity of the solvent, the more the 1,2 bonds of the polybutadiene. Polybutadiene having high 1,2 content has high glass transition temperature (Tg), and it is believed that this polymer can be used effectively in tires demanding high grip performance. However, if we are to place importance on high abrasion resistance, polybutadiene having low Tg and low 1,2 bond contents is demanded. In particular, products with low 1,2 bond content can be effectively polymerized in solvents having low polarity. In order to improve the "Functionality Introduction Ratio" of polymer, the efficient promotion of the first reaction of the mechanism shown in Scheme I (Benzyl hydrogen of TD being replaced by lithium) becomes the key. With the aim of improving the reaction efficiency of TD and BuLi in a low THF concentration, a study was made on the addition of Ν,Ν,Ν,'Ν'-Tetramethylethylenediamine (TMEDA).


73

I 8

1 7

I 6

I 5

I 4

I 3

I 2

I 1

U

TD

0

6 in ppm Figure 9. 1 H - N M R spectrum of the reaction products obtained in C H X / T H F =75/25.

->

elution time

Figure 10. HPLC curve of the reaction products obtained in CHX/THF = 88/12.

74


Table III. Formation ratio of TD-Li and BuLi in mixed solvents of cyclohexane and tetrahydrofuran

ft

M

Solvent C H X / THF

TD-Li (TD-Li + BuLi)

Functionality Introduction Ratio

(wt/wt)

(molar ratio, %)

(%)

100/0 95/5 88/12 75/25 69/31 39/61 18/82

0 21 37 84 88 89 88

0 15 35 93 90 92 91

(a) ; calculated from the peak areas of high performance liquid chromatography on the reaction products of TD/BuLi and benzylchloride (b) ; values obtained by Ή-NMR of polybutadiene

In Figure 11, the relation between the CHX/THF ratio of a solvent and "Functionality Introduction Ratio (value obtained by ^H-NMR)" when the addition amount of TMEDA is varied, is shown. As we anticipated, by adding TMEDA, the functionality improved significantly. Furthermore, it was found that the more TMEDA is added, the greater the improvement of functionality. In Fig. 12 the relation between the 1,2 bond content in the polymer and the "Functionality Introduction Ratio" (value obtained by ^H-NMR) is shown. In a system where TMEDA is added, we found that polybutadiene having aromatic tertiary amino groups efficiently introduced can be obtained at comparatively low 1,2 bond content. Conclusion A study was made on a new initiator Ν,Ν-dimethylaminobenzyllithium (TD-Li) with the purpose of improving functionality of the initial chain end in anionic polymerization. Experiments on polymerization of 1,3-butadiene using this initiator were made, and the following results were obtained. 1) . After reacting TD with BuLi in THF, when polymerization of 1,3-butadiene was conducted, polymers were obtained at a high conversion rate. From H - N M R , Infrared Absorption and GPC analyses, it was found that the polymer contained an aromatic tertiary amino group which is the residue of the TD-Li initiator. 2) . The "Functionality Introduction Ratio" of the polybutadiene obtained by using the initiator TD/BuLi, was influenced by the polarity of the solvents used in the polymerization. The higher the polarity, the higher the "Functionality Introduction Ratio." 3) . Solvent effect would be dominant in the step of the initiator synthesis. Namely, based on the results of the model reactions, in high polarity solvents the X


75

0C\ 1-Fk τ J

100 (Ο 80

»? |.2 •4-·

Π

?

60

·π

40

Ο ο

r2

ï

TMEDA/Li=1.0 TMEDA/Li=0.3 TMEDA/Li=0.1 TMEDA/Li=0.0

•

20

—

0 100/0

8

0

/

2

0

60/40

4

0

/

6

0

20/80

Q

/

1

0

0


Figure 11. Relationship between "Functionality Introduction Ratio" and polarity of solvent.

©

0

20

40

60

80

TMEDA/Li = 1.0

Ο

TMEDA/Li = 0.3

Ο

TMEDA/Li = 0.1

•

TMEDA/Li = 0.0

100

Functionality Induction Ratio (%)

Figure 12. Relation between "Functionality Introduction Ratio" and vinyl content of polymer.


76 reaction between TD and BuLi succeeded efficiently, and a benzyllithium type compound (TD-Li) was formed. The said compound acted as the initiator, and the polymerization of butadiene proceeded. On the other hand, in a solvent having low polarity where TD-Li formation was incomplete, the polymerization of butadiene was initiated by the mixed system of TDLi and BuLi. 4). In the case where TMEDA is added to the reaction system, the "Functionality Introduction Ratio" is improved. Consequently, we found that polybutadiene having aromatic tertiary amino groups efficiently introduced can be obtained at comparatively low 1,2 bond content REFERENCES 1. N. Ohshima, F. Tsutsumi and M. Sakakibara, IRC Kyoto, Oct. 1985, 16A04. 2. F. Tsutsumi, M. Sakakibara and N. Ohshima, Rubber Chem. Technol. 1990, 63, 8. 3. T. Tadaki, F. Tsutsumi, M. Sakakibara and I. Hattori, 146th. Meeting of Rubber Div. ACS, Pittsburgh 1994, paper No. 64. 4. N. Nagata, T. Kobatake, H. Watanabe, A. Ueda and A. Yoshioka, Rubber Chem. Technol. 1987, 60, 837. 4. Ν. I. Nikolayev, N. M. Geller, B. A. Dolgoplosk, V. N. Zgonnik and V. A. Kropachev, Polym. Sci. USSR 1963, 4,1529. 5. P. A. Vinogradov and Ν. N. Basseva, Polym. Sci. USSR 1963, 4,1568. 6. A. C. Angood, S. A. Hurley and P. J. T. Tait, J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 2777. 7. T. W. Bethea, W. L. Hergenrother, F. J. Clark and S. B. Sarkar, "Novel Tin -containing Elastomers Having Reduced Hysteresis Properties" IISRP, 1994. 8. D. Morero, A. Santambrogio, L. Porri and F. Ciampelli, Chim. Ind. (Milan), 1959,41 (8), 758.

Polymerization of Butadiene with Novel tert-Amino Group Initiator

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